Field effect transistors play a central role in all areas of electronics. In order to adapt them to suit particular applications, it has been necessary to make them lighter and more flexible. The development of semiconducting and conducting polymers has made it possible to produce organic field effect transistors, all parts of which, including the semiconductor layer as well as the source, drain and gate electrodes, are fabricated from polymeric materials.
However, in the production of organic field effect transistors a plurality of organic layers have to be patterned one on top of the other in order to obtain an OFET of normal construction, as shown in FIG. 1. This is possible only to a very limited extent using conventional photolithography which is actually used for patterning inorganic materials. The operations normally involved in photolithography dissolve or attack the organic layers and therefore make them unusable. This occurs, for example, when a photoresist is spun on, developed and stripped off.
This problem has been solved using an organic field effect transistor as described in Applied Physics Letters 1998, page 108 et seq. A polyaniline-coated polyimide film is used as the substrate. In this first polyaniline layer, the source and drain electrode are formed by irradiation through a first mask. In this first layer, a semiconductor layer of polythienylenevinylene (PTV) is also formed, on which polyvinylphenol is then crosslinked using hexamethoxymethylmelamine HMMM. This layer is used as the gate dielectric and as an insulator for the next layer and the interconnects. A further polyaniline layer is finally formed thereon in which the second layer of interconnects and the gate electrode is defined by patterning. The vertical interconnects are produced mechanically by punching pins through the layers.
The above method prevents previously applied layers from being dissolved or otherwise damaged. However, it has been shown that in particular the last operation for forming the vertical interconnects (otherwise known as vias) does not permit the fabrication of complex circuits.
Applied Physics Letters 2000, page 1487 describes how this problem can be solved by providing low-resistance vias in the field effect transistor structure by means of photopatterning of photoresist material. To this end, another design of OFET, namely a so-called “bottom gate” structure, is regarded as indispensable. If a “top gate” structure of the same composition were produced, this would result in unacceptable contact resistances in the order of M.
However, the construction and the operations for patterning this OFET with bottom gate structure are complex, making it impossible to manufacture particularly complex circuits economically.